A VSC is a controllable voltage source, with an input connected to at least one capacitor functioning as a DC voltage source. At the outputs, the converter is operable to create a variable AC voltage. This is achieved by connecting the voltages of the capacitor or capacitors directly to any of the converter outputs using the switches in the VSC. In converters that utilise pulse-width modulation (PWM), the input DC voltage is normally kept constant when creating output voltage waveforms that in average are sinusoidal. The amplitude, the frequency and the phase of the AC voltage can be controlled by changing the switching pattern.
As discussed in WO 2010/102666 A1, a VSC may be formed by converter cells connected in series. This improves the smoothness of the AC output at a given switching frequency and for given filtering. The cells may be single-phase full-bridge converters, typically referred to as chain-link cells. A chain-link based converter comprises a number of series-connected cell modules, each cell comprising a capacitor, besides the switches. The number of cells in series in each phase is proportional to the AC voltage rating of the system and may consequently include a large number of cells for high AC voltage systems.
In order to adapt a VSC of this general type to a given AC current rating, two main approaches are known in the art. One option is to arrange sequences of serially connected cells in parallel, wherein each sequence provides a share of the total current without exceeding its own current rating. Due to the complex dynamics of the respective sequences, advanced control is needed in order to avoid imbalances between parallel sequences. Another option is to design a chain-link cell, the critical components of which withstand the required switching current, wherein copies of the cell are to be arranged into a single sequence in accordance with the required switching voltage. In order for the resulting VSC to meet reasonable quality expectations, in particular regarding energy efficiency, this approach leaves the designer in a dilemma regarding the size of the range of cell types to be offered. In a small range, there is statistical certainty that some cells will be installed in applications were they are under-utilized while still occupying a large footprint. Accordingly, the cost for a VSC according to this approach may vary unexpectedly in relation to the electric power subject to switching. A large range of cell types, on the other hand, will necessitate an extensive effort to develop, high costs to manufacture (e.g., the number of common parts may be small) and difficulties related to inventory management.